Mosquitoes are well recognized as the most important arthropod vectors of disease-causing pathogens. Thus, studies on processes with potential to disrupt mosquito development or disease transmission are of biomedical importance. This application is a resubmission request to build on recent results showing that the gut micro biota of Aedes aegypti and other mosquito species are essential for development. It has long been known that the gut harbors bacteria but the role these microbes play in mosquito biology is largely unclear. Here we report preliminary data showing that the community of bacteria in the digestive tract of the mosquito Aedes aegypti is relatively simple but also exhibits distinct changes during development. Our results further show that bacteria-free (axenic) Aedes aegypti cannot develop, while recolonization of the gut by several gut community members and the non-community member Escherichia coli rescue development. Comparative data with other species yield similar results, which strongly suggests a fundamental, but heretofore unrecognized, dependence by most if not all mosquitoes on their gut microbiome for development. Recent studies identify bacterial genes with roles in developmental rescue as well as insights into how bacteria affect mosquito physiology. To further advance these results, we propose three specific aims. 1. Complete characterization of the gut micro biota in our laboratory cultures of Ae. aegypti and select other species, and conduct parallel experiments in field collected mosquitoes. 2. Further characterize E. coli mutants defective in rescuing mosquito development. 3. Analyze how bacteria affect mosquito physiology. Expected outcomes of our work will enhance understanding of the mosquito gut microbiome and its functions. Our study will impact the field of vector biology by providing information on the largely unknown but critical role gut microbes play in mosquito development. Our study will also provide information of potential importance in manipulating mosquito development as a strategy for vector control.